Update tests.
Reason: it turns out that the native output generated by
pretty-simple isn't always readable by the native reader.
According to https://github.com/cdepillabout/pretty-simple/issues/99
it is not a design goal of the library that the rendered values
be readable using 'read'. This makes it unsuitable for our
purposes.
pretty-show is a bit slower and it uses 4-space indents
(non-configurable), but it doesn't have this serious drawback.
Previously we used our own homespun formatting. But this
produces over-long lines that aren't ideal for diffs in tests.
Easier to use something off-the-shelf and standard.
Closes#7580.
Performance is slower by about a factor of 10, but this isn't
really a problem because native isn't suitable as a serialization
format. (For serialization you should use json, because the reader
is so much faster than native.)
Closes#4284.
Headers with the corresponding tags should not appear in the output.
If one or more of the specified tags contains a non-tag character
like `+`, Org-mode will not treat that as a valid tag, but will
nonetheless continue scanning for valid tags. That behavior is not
replicated in this patch; entering `cat+dog` as one of the entries in
`#+EXCLUDE_TAGS` and running the file through Pandoc will cause the
parser to fail and result in the only excluded tag being the default, `noexport`.
